The black matrix which is applied to the interior surface of color picture tubes improves the contrast performance of color television pictures. On their interior surface, these color picture tubes which are also referred to as black matrix tubes, have a striped structure characterized by alternating light-absorbing stripes and light-transmitting stripes. As a rule, these stripes are arranged vertically, as viewed from the position of an observer. The width of the absorbing and transmitting stripes ranges from 150 to 190 .mu.m.
To date, such a matrix is produced in a multistep process using negative photoresist materials. The negative photoresist materials are coated on the interior surface of the picture tube and dried. This is followed by stripe exposure using a mask, and a developing process, after which the non-exposed negative photoresist stripes remain. Subsequently, a colloidal suspension of carbon black or graphite is coated on the interior surface of the color picture tube.
Then, in a further processing step, the remaining negative photoresist stripes, together with the graphite or carbon black coating are removed so that, when the process is completed, stripe-shaped carbon black or graphite layers, alternating with light-transmitting stripes, are formed.
Such a two-step process (negative-positive process) involves high technological input and requires providing both a photoresist and a graphite or carbon black dispersion. The relatively large number of single processing steps which, in particular, may affect the peripheral zones of the stripes, may also give rise to the phenomenon of blurring in the peripheral zones of the stripes.
EP 91 163 describes a positive-working composition which is used for the production of a light-absorbing matrix in color picture tubes. The positive-working photoresist composition essentially consists of hydrolyzed poly(vinyl alcohol), ammonium trioxalatoferrate, ferric nitrate, and finely dispersed graphite or carbon black, with the mentioned ingredients being dispersed in water. This suspension is coated on the interior surface of the screen of the color picture tube, dried and exposed to UV radiation as appropriate for the structure required.
DE 31 42 261 A1 reports a process for producing a black matrix layer between the luminous surfaces on the inside of the troughs of color picture tubes. Therein as well, a photosensitive lacquer (positive resist) is coated on the inside of the screen after cleaning. The lacquer contains poly(vinyl alcohol) and poly(methyl acrylate) as film-forming components as well as a dichromate sensitizer, with graphite being contained as black pigment. After appropriate exposure and development the exposed areas are completely removed to form a positive image of the black matrix layer.
As this photoresist composition is a positive-working one, it is sufficient to remove the areas rendered water-soluble by the exposure from the surface in a subsequent developing process.
It appeared, however, that the known positive photoresist compositions used in the production of a black matrix suffer from various drawbacks. The solubility difference between the light-absorbing portions remaining on the glass surface and the areas to be removed is very small, so that removal of the non-exposed layer areas can be observed as well, resulting in a deterioration of absorbing properties and edge definition of the black matrix. This relatively small differentiation in solubility becomes even more unfavorable if light from the exterior inadvertently interferes with the process.
The invention is based on the object of providing a black matrix produced using a positive photoresist composition which permits good capability of structuring between exposed and non-exposed areas, and reducing the process for producing the black matrix to just a few technological steps.